It has long been known that oil may be extracted by heat from various extensive deposits of porous minerals, known by the generic term "oil shale," which are permeated by a complex organic material called "kerogen." Upon application of heat, the kerogen is converted to a complex mixture of hydrocarbons and hydrocarbon derivatives which may be recovered from a retort as a liquid shale oil product.
The shale oil so recovered contains various compounds of oxygen, sulfur, nitrogen and heavy metals combined with the carbon and hydrogen of desired hydrocarbon products. For the most part, the components of shale oil have boiling points in the upper levels of boiling ranges of natural petroleum, say upwards of 50% of the total boiling above about 750.degree. F. Such high boiling fractions are unsuited to use in premium grade fuels. Even after requisite removal of sulfur, nitrogen and metals, these fractions must be processed further or sold as the cheaper grades of heavy fuel such as No. 6, Bunker Oil, etc.
In addition, the shale oil contains a relatively high proportion of straight chain aliphatic compounds of high molecular weight typical of hydrocarbon waxes. These long carbon chain compounds tend to crystallize on cooling of the oil to an extent such that the oil will not flow, hence may not be pumped or transported by pipeline. The temperature at which such mixture will not flow is designated the "pour point," determined by standarized test procedures.
The pour point problem can be overcome by techniques known in the art for removal of waxes or conversion of those compounds to other hydrocarbons which do not crystallize at ambient temperatures. An important method for so converting waxy hydrocarbons is shape selective cracking or hydrocracking utilizing principles described in U.S. Pat. No. 3,140,322 dated July 7, 1964. Zeolitic catalysts for selective conversions of wax described in the literature include such species as mordenite, with or without added metal to function as a hydrogenation catalyst.
Particularly effective catalysts for catalytic dewaxing include zeolite ZSM-5 and related porous crystalline aluminosilicates as described in U.S. Pat. No. Re. 28,398 (Chen et al.) dated Apr. 22, 1975. As described in that patent, drastic reductions in pour point are achieved by catalytic shape selective conversion of the wax content of heavy stocks with hydrogen in the presence of a dual-functional catalyst of a metal plus the hydrogen form of ZSM-5. The conversion of waxes is by scission of carbon to carbon bonds (cracking) and production of products of lower boiling point than the waxes. However, only minor conversion occurs in dewaxing. For example, Chen et al. describe hydrodewaxing of a full range shale oil having a pour point of +80.degree. F. to yield a pumpable product of pour point at -15.degree. F. The shift of materials from the fraction heavier than light fuel oil to lighter components was in the neighborhood of 9% conversion.